Transmission systems with lower output power and range operate on different frequency bands worldwide. In Europe, this is the ISM (Industrial Science Medical) band around 433.92 MHz and the short range device (SRD) band between 869 MHz and 870 MHz, for example. By contrast, in the USA, use is frequently made of the ISM band and the very wide band around 915 MHz. There are further bands around 315 MHz and around 390 MHz, inter alia.
Many applications, such as telemetry, for example for tire pressure measurement systems (TPMS) or remote keyless entry (RKE), can be operated license-free in the bands therein. However, some manufacturers of corresponding appliances often use only narrowband frequency ranges which are used exclusively for the respective applications.
For such narrow frequency ranges, it is possible to use receivers with narrow band filters, for example on the basis of quartz filters. If a plurality of such applications using closely adjacent but different channels for these frequency bands are now used in a motor vehicle, for example, each of these applications sometimes requires a dedicated reception filter which is in turn of correspondingly narrowband design for the respective channel. In order to separate the channels, it is then possible to use switches.
If the aim is to dispense with the switches in a receiver, however, a front end filter whose passband covers all the desired channels is required. On account of the bandwidth of the individual channels and also tolerances which need to be taken into account, a reception filter would then be required whose bandwidth cannot be implemented using a quartz filter, or can be implemented only with worsening properties. As the bandwidth increases, the impedance of quartz-based SAW (Surface Acoustic Wave) filters rises, and the matching stability also suffers at higher impedances.